The present invention relates to a constant current driving device for constant current driving of a plurality of elements, for example light emitting diodes (LEDs) connected in series with each other by a pulse width modulation constant current driving circuit, a backlight light source device driven by the constant current driving device, and a color liquid crystal display device.
As is typified by liquid crystal panels and plasma display panels (PDPs), there has recently been a trend toward thinner displays. Among the displays, many displays for mobile use are liquid crystal panels, which are desired to have faithful color reproducibility. While a mainstream backlight for liquid crystal panels is a CCFL (Cold Cathode Fluorescent Lamp) type using a fluorescent tube, mercury-less backlights have been requested from an environmental point of view. Light emitting diodes and the like are considered to be promising as a light source to replace the CCFL.
Generally, a display using light emitting diodes as display pixels requires an X-Y addressing driving circuit for the pixels to perform matrix driving of the light emitting diodes. The X-Y addressing driving circuit selects a light emitting diode at a position of a pixel desired to be lit (addressing), and adjusts brightness of the light emitting diode by varying a lighting time by pulse width modulation (PWM), whereby a display screen with a predetermined gradation is obtained. Therefore the driving circuit is complex and requires a high cost (see Japanese Patent Laid-Open No. 2001-272938, for example).
Light emitting diodes also have life. Failure of an individual element is roughly divided into three types: (1) a failure in an OPEN mode in which a disconnection occurs; (2) a failure in a Short mode in which a short circuit occurs; and (3) a mode that is neither of the above modes and in which a decrease in light quantity occurs.
To detect these failures requires employment of a method of driving each LED element by an independent driving circuit and construction of a system for feeding back a state of operation of each element at all times, which increases cost and is thus difficult to realize in an actual apparatus.
There are image displays using light emitting diodes as individual light emitting pixels. In matrix type driving in this case, there has conventionally been no system having a function of individually determining a failure of each of light emitting diode elements as described above and further eliminating the failure.
In a case where light emitting diodes are used as a backlight for a liquid crystal display, power to each light emitting diode is high, and the number of light emitting diodes is relatively small. Therefore, when a part is unlit due to a failure, unevenness or the like occurs, which is not comfortable to the eye. A matrix driving LSI or the like for high power driving in LED driving devices for a lighting purpose has not been created, and is practically disadvantageous in terms of cost. Therefore a series connection form is used. However, in the series connection form, when a failure occurs in an individual light emitting diode and the failure is a disconnection, all light emitting diodes in the row are not lit, thus causing considerable color unevenness.